fix [core] trace level shows the first kb of the payload before being decoded (#534)

Add a log statement in the Lambda loop, before calling the user's
handler to show the raw payload before any attempt to decode it.

This was available in Runtime v1 and is now ported to v2.
This fixes
#404

### Motivation:

This is useful when handling custom event and there is a Decoding error.
It allows to see the exact payload received by the handler before any
attempt to decode it.

### Modifications:

Add a log.trace statement with metatadata. Metadata are computed only
when the log level is trace or below.

### Result:

```
2025-07-21T08:58:33+0200 trace LambdaRuntime : Event's first bytes={"name": "me", "age": 50} aws-request-id=769127502334125 [AWSLambdaRuntime] sending invocation event to lambda handler
```

---------

Co-authored-by: Copilot <[email protected]>

Author: sebsto

Sources/AWSLambdaRuntime/Lambda.swift

@@ -44,6 +44,24 @@ public enum Lambda {
                 let (invocation, writer) = try await runtimeClient.nextInvocation()
                 logger[metadataKey: "aws-request-id"] = "\(invocation.metadata.requestID)"
 
+                // when log level is trace or lower, print the first Kb of the payload
+                let bytes = invocation.event
+                let maxPayloadPreviewSize = 1024
+                var metadata: Logger.Metadata? = nil
+                if logger.logLevel <= .trace,
+                    let buffer = bytes.getSlice(at: 0, length: min(bytes.readableBytes, maxPayloadPreviewSize))
+                {
+                    metadata = [
+                        "Event's first bytes": .string(
+                            String(buffer: buffer) + (bytes.readableBytes > maxPayloadPreviewSize ? "..." : "")
+                        )
+                    ]
+                }
+                logger.trace(
+                    "Sending invocation event to lambda handler",
+                    metadata: metadata
+                )
+
                 do {
                     try await handler.handle(
                         invocation.event,